1. Field of the Invention
The present invention relates to a telephoto lens optical system and, more particularly, to an inner focus type telephoto lens optical system.
2. Related Background Art
As a conventional focusing system of a telephoto lens, a system for attaining focusing by moving the entire lens system in the optical axis direction is used.
However, Japanese Laid-Open Patent Application Nos. 52-55639 and 57-165809, and the like proposed a telephoto lens optical system, which comprises three lens units, in the following order from the object side, a positive lens unit, a negative lens unit, and a positive lens unit, and attains focusing to a short-distance object by moving the negative lens unit toward the image side.
However, in the telephoto lens system disclosed in Japanese Laid-Open Patent Application No. 52-55639, each lens unit itself is required to be sufficiently subjected to aberration correction, so as to suppress variations in aberration upon focusing. However, since the number of lenses is reduced to achieve a compact system, the degree of freedom upon aberration correction is lowered, and as a result, imaging performance at a photographing magnification of .times.0.1 or higher is poor.
In the telephoto lens system disclosed in Japanese Laid-Open Patent Application No. 57-165809, the lens surface, closest to the object side, in the second lens unit, which contributes to correction of variations in aberration, has a concave surface facing the object side. For this reason, the lens surface, closest to the object side, in the second lens unit falls outside the range of a minimum angle of deviation for Rund rays in convergent light from the first lens unit having a positive refractive power. Therefore, the second lens unit corrects spherical aberration excessively in the positive direction, and a variation in spherical aberration corresponding to a change in incident angle of Rund rays is generated upon focusing. As a result, preferred imaging performance cannot be obtained.
Note that the Rund rays are light rays, which are parallel to the optical axis and are most separated from the optical axis, in a light beam incident from an infinity object, i.e., maximum height light rays.